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Perpetual motion
Perpetual motion is motion that continues indefinitely without any external source of energy.〔(【引用サイトリンク】title=Dictionary - Definition of perpetual motion )〕 This is impossible to ever achieve because of friction and other sources of energy loss. A perpetual motion machine is a hypothetical machine that can do work indefinitely without an energy source. This kind of machine is impossible, as it would violate the first or second law of thermodynamics.〔〔
These laws of thermodynamics apply even at the grandest scale: for example, the motion or rotation of celestial bodies such as planets may appear perpetual, but are actually subjected to many forces such as solar winds, interstellar medium resistance, gravitation, thermal radiation and electro-magnetic radiation, and will eventually end.
Thus, machines which extract energy from seemingly perpetual sources will not operate indefinitely, because they are driven by the energy stored in the source, which will eventually be exhausted. A common example is devices powered by ocean currents, whose energy is ultimately derived from the Sun, which itself will eventually burn out. Machines powered by more obscure sources have been proposed, but are subject to the same inescapable laws, and will eventually wind down.
==History==
(詳細は
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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